1. Field of the Invention
The present invention relates to an apparatus for separating disk-type workpieces, in particular semiconductor wafers, from a wafer stack. It furthermore relates to a method of separating disk-type workpieces.
2. The Prior Art
The production of single-crystal or polycrystalline semiconductor wafers, for example composed of silicon or of compound semiconductors such as gallium arsenide or indium phosphide, requires the sawing of rod-shaped or block-shaped single-crystal or polycrystalline workpieces. In this operation, use is made, in particular, of wire frame saws with which up to several hundred thin single-crystal or polycrystalline wafers are simultaneously cut from an ingot or block of the semi-conductor material in one sawing operation. This then produces, after the sawing operation, a stack of these thin wafers which may be contaminated with sawdust or sawing fluid and which then have to be removed individually and introduced into a product carrier (wafer magazine, tray) for further treatment. This operation is normally carried out by hand.
In a similar manner, wafer populations existing in stacked form are produced on sawing single-crystal workpieces, for example ingots, into wafers and may also be encountered in further phases of the treatment process, for example, during lapping, etching or polishing steps. In the stack, the wafers may exist in essentially clean form or appreciably contaminated form, for example, due to sawdust, abrasion or oil.
The separation of the wafers is necessary in order for example to be able to carry out a further treatment of the surface. Such treatment operations can only be carried out completely and cleanly if both surfaces are readily accessible for the treatment agent, for example cleaning, coating or etching solutions, polishing or lapping tools or gases for reacting with the surface. For this purpose, the individual wafers are introduced, for example, into processing trays.
In particular, if the wafers have a small thickness of usually only up to approximately 200 .mu.m and consequently have a great fragility, separation by hand is very difficult and unpleasant work. The low mechanical strength of the wafers results in losses due to breakage. Since extremely high requirements in relation to the cleanliness and perfection of the surface are imposed on semiconductor material, the performance of this work by hand is also a reason for contamination and damage, for example, due to scratches, which should be avoided.
Attempts at automation have hitherto not been successful. Thus, removing the wafers from the stack by mechanical means (such as, for instance, vacuum suction devices) fails because of the adhesive forces between the wafers, which frequently cause breakage losses. In addition, a mechanical contact with the wafer, which results in contaminants and defects on the wafer surface, is necessary.
Contaminants of the type described above on the wafer surface, for example as a result of sawdust, abrasion or oil, are a further problem with which the previous systems did not solve. Thus, in the case of dried wafers, contaminants result in sticking together and, consequently, in difficulties occurring during separation while moist or sticky wafers contaminated with solid constituents result in lack of cleanliness and, consequently, unreliability of the separating equipment. In all cases, the probability of damage is greater in the case of contaminated wafers. In addition, the opinion was held that the adhesive forces between the wafers are too strong for an automatic separation to be carried out, since these forces can even be utilized for some processes, for instance, bonding.